1. Field of the Invention
The invention concerns a device and a method for generating during periods of at least several seconds a high mass flow of unpolluted high enthalpy air at a velocity of approximately Mach 6 (the expression "hypersonic flow" is often employed in relation to such velocities).
It is more particularly, although not exclusively, directed to generating a high mass flow of unpolluted air in a wide range of flight conditions over simulation periods sufficient for studies of the steady state operation of supersonic ram jets of the type usually referred to as "scramjets".
The invention is also applicable to studying the behavior of materials in hypersonic airflows, for example.
2. Description of the Prior Art
The use of a scramjet would appear to be the inevitable choice for the propulsion of hypersonic vehicles of the future, whether launch vehicles, missiles, aircraft, etc.
Testing scramjets on the ground entails simulating high Mach numbers and therefore generating high enthalpy airflows.
Existing scramjet test facilities in Europe are restricted to simulating Mach numbers below 6.5, i.e. to a temperature of around 1 800K. of the air to be injected into a motor under test. However, Mach 6.5 is at the very lowest end of the range of operation of scramjets which are designed to operate at velocities up to Mach 12 and above.
Attempts have been made to adapt existing test facilities to raise the aforementioned limit of Mach 6.5 but the results have been less than perfect (causing significant pollution of the air).
U.S. Pat. No. 3,744,305 teaches, for generating a flow of air at hypersonic velocities in the order of Mach 8 to Mach 10, pressurizing an enclosure with air at a pressure of 100 to 200 atmospheres and at a temperature of 5,500.degree. R (3,060K) to 6,000.degree. R (3,336K) and then injecting a gas adapted to react exothermically with the pressurized air so as to maintain the pressure and the temperature of the air as it escapes at high velocity from the enclosure. This chemical reaction is incompatible with obtaining unpolluted air.
The main difficulty is to raise the air to be fed to a motor under test to a high temperature without polluting it and while approximating as closely as possible actual flight conditions (in particular, over a duration of at least several seconds).
Conventional scramjet test facilities which heat the air by thermal exchange or by combustion in a burner combined with re-oxygenation cannot be used above Mach 6.5.
Accumulator and continuous type heaters are at the limits of existing technology; hydrogen burners, for example, supply a mixture of air and water vapor in which the quantity of water considerably disrupts combustion beyond Mach 6.5. with the result that combustion studies are not reliable.
The use of plasma torches may seem feasible but, apart from their high cost, they have the drawback of causing dissociation of the air molecules which disturbs the kinetics of the combustion chemistry.
As for so-called "shock tunnel" devices, they can generate high enthalpies for only a few milliseconds, totally insufficient for steady state combustion studies.
One example of a shock tunnel generating very short air blasts is described in U.S. Pat. No. 3,505,867.
Thus it has become clear that new propulsion unit test facilities must be developed if scramjets are to be developed further.
An object of the invention is to generate at moderate cost unpolluted airflows at high temperatures (at least 1,800K) for periods of several seconds to enable simulation of stable hypersonic combustion at Mach numbers of 6.5 and above, possibly as high as Mach 8 or above.
To this end the invention teaches that the air to be fed to the motor under test should be heated by compressing it.
Compressing a given quantity of air is known to increase its temperature.
If the compression is carried out under adiabatic conditions, that is to say without exchange of heat with the surrounding environment, and assuming that air is a perfect gas in the usual thermodynamics sense of the term, the laws of thermodynamics enable us to write: ##EQU1## in which: To and Po are the temperature and pressure of the quantity of air in question before compression,
T and P are the temperature and pressure of this quantity of air after compression, and PA0 .gamma. is the ratio (assumed constant) of the calorific capacities of this air at constant pressure and volume, respectively. PA0 r is a constant characteristic of the gas, expressed in energy per degree and per unit mabs (the value of this constant is approximately 287 J/K/kg for air).
Taking .gamma.=1.3, the final temperature T obtained as a result of compression is therefore expressed as a function of the initial temperature To and the compression ratio (P/Po) by the equation: ##EQU2##
This equation shows that if meaningful results are to be achieved a high compression ratio or a high initial temperature is required.
To compress the feed air the invention teaches the use, without the interposition of any screening means, of a high pressure air blast such as can be delivered by a conventional blow down type wind tunnel. The investment costs of the invention are reduced commensurately.
One example of a blow down type wind tunnel is described in French Patent No. 2,633,677 but is restricted to temperatures of 40.degree. to 90.degree.C. and to velocities of 200 to 300 m/s.
The performance that can be achieved by using an air blast to compress the feed air without the interposition of any screening means is improved if there is no (or little) mixing between the air blast and the feed air, i.e. if the area of the interface between these masses of air is small. To this end, according to the invention the mass of feed air is elongate with a circular, polygonal or other cross section and the air blast is applied to one side of the mass of feed air while confining it laterally. To this end the mass of feed air is enclosed in a tube or tunnel with thermally insulative walls, that is to say a quasi adiabatic tube or tunnel.
For a given mass of feed air at given pressure and temperature the surface area of the interface and the degree of mixing between the air masses is inversely proportional to the length/cross section ratio of the mass of feed air (i.e. the tunnel).
However, thermal exchange between the feed air and the confining tunnel containing it is directly proportional to this ratio so that the more elongate the shape the more marked the departure from adiabatic compression.
A compromise is therefore required in respect of the length/cross section ratio of the mass of feed air in order to minimize the mixing of the air masses (and therefore the area of the interface with the air blast) and thermal exchanges (the area of the interface with the confinement tunnel).
The thermal losses are reduced if the time of thermal exchange at the boundaries of the feed air is short (and therefore compression is fast and the flow rate of the air blast is high) and also if the feed air is used quickly after it is compressed.
The compressed and therefore heated feed air is itself provided in the form of a blast of mass flow rate d.sub.r and of duration t.sub.r so that its total mass is given by the equation: EQU M.sub.r =d.sub.r. t.sub.r
The volume required to store this mass of feed air at pressure Po and temperature To is deduced using the laws of thermodynamics for perfect gases: EQU V=r.multidot.(To/Po).multidot.M.sub.r
in which:
This equation shows that the storage volume in the tunnel is inversely proportional to the storage pressure Po, which mitigates against a high compression ratio since the maximum pressure that can be obtained after compression is limited by the maximum pressure of the compression air blast.